How to change graphics card?

I have the lenovo y510p and I was wanting to know how to change or upgrade the graphics card. Right now I have the GeForce gt 750m w/ 2gb. My laptop has the ultrabay so I can take the one of the graphics card out and put in a cd rom. I heard that you could put in a better graphics card but only in the family of the graphic card. Since I have the GeForce gt 750m I wanted to change it to the highest graphic card in its family wich is the GeForce GTX 780m. Here is the link for the graphics card and its family.
http://www.geforce.com/...80m/videos

So would I be able to buy a better graphics card and replace it?

You would have to check with the Lenovo web site, what is supported by your model. In many cases, you have to buy vendor specific upgrades because of the BIOS and laptop design.

You can't change your graphics card in a laptop because the graphics chip is built in.

Pretty much, no. The graphics chips in laptops are normally built right on to the motherboard. Even if they are on a card that could be replaced, the card has to be made specifically to support that make and model of laptop exactly and are very expensive. Whatever you buy in your laptop is what your stuck with till you buy a new one.